1. Field of the Invention
The invention relates to an electric power steering apparatus.
2. Description of Related Art
An electric power steering apparatus (EPS) having a motor as a drive source is known as a power steering apparatus for a vehicle. An EPS exhibits a greater degree of layout freedom and lower energy consumption than a hydraulic power steering apparatus, and therefore EPS apparatuses are being used ever more widely, irrespective of vehicle type, vehicle classification, and so on.
In a vehicle having a battery (and an alternator serving as a charger) as a main power supply, however, there is a limit to a magnitude of the power output from the battery. Therefore, in a large vehicle, an assist force of the EPS may be insufficient in conditions where a large amount of power is required, such as when a steering operation is performed while stationary (during so-called stationary steering), and as a result, a steering wheel operation may not be performed smoothly.
In response to this problem, a power supply device in which an auxiliary power supply is provided separately to the main power supply has been proposed. In this device, power supply using the auxiliary power supply is performed in addition to normal power supply using only the main power supply, and therefore power can be supplied in a condition where the main power supply and the auxiliary power supply are connected in series. For example, Japanese Patent Application Publication No. 2007-223510 (JP-A-2007-223510) discloses this type of EPS including a main power supply and an auxiliary power supply.
Further, since the power supply of this type of EPS handles large amounts of power, faults (short circuit faults, for example) in the output of the power supply must be detected, and when an abnormality occurs, a current supplied to an output destination of the power supply must be shut off to prevent damage to the output destination. A shunt circuit in which a shunt resistor is disposed on an output circuit of a power supply so that an overcurrent caused by a short circuit is detected in accordance with variation in a potential difference between respective ends of the shunt resistor has been proposed as means for detecting a fault in the output of the power supply (Japanese Patent Application Publication No. 2007-135374 (JP-A-2007-135374), for example).
However, component sizes and component numbers are being reduced in vehicles with a view to improving fuel efficiency and reducing cost, and similar demands are being made of the EPS described above. In an EPS that handles a large amount of power such as that described above, however, the shunt circuit is provided on the output circuit of the power supply in order to detect faults in the output of the power supply device, and therefore the size of the shunt circuit increases in proportion to the magnitude of the output of the power supply, making it difficult to reduce the size and cost (and the number of components) of the power supply.